Frequently, lateral exits have to be made from existing wellbores to access additional production. This typically involves orienting and anchoring a whipstock to direct one or more mills laterally to make an elongated opening in the casing. This technique has been around for a long time and has been the target of efforts to optimize it. One of the advances made was to produce the window in a single trip by running in a whipstock with a series of mills. The initial mill, known as the window mill was secured to a lug near the top of the whipstock and that connection sheared with the application of weight before milling. Behind the window mill were additional mills, known as string mills or watermelon mills. Watermelon mills are so named due to their more rounded profile. When used in this application, either term, unless specifically modified, is intended to cover mills that have straight or curved bearing structure or blades. The rounded or straight larger diameter could also have a combination of bearing and cutting structures. Illustrative of an early effort to make a window in a single trip was Jurgens U.S. Pat. No. 5,109,924. This reference featured a window mill 6 close coupled to the lower watermelon mill 14 and a reduced diameter flexible joint 15 located right above the lower watermelon mill 14 so that the rigid assembly of the window mill 6 and the watermelon mill 14 could make the bend onto the whipstock ramp which was usually in the order of about 2°. The rigidity of three close coupled mills was too great to make this bend, requiring a reduced diameter, more flexible joint between the two upper mills and the two lower mills. Additionally the two lower mills were nowhere close to the inside or gauge diameter of the tubular in which the assembly was anchored. In that manner the flexible joint and reduced lower mill diameters allowed the assembly to avoid binding as it turned onto the whipstock and eventually exited into the formation.
Eventually, the market evolved to demand not only to mill the window, but to continue and drill the lateral where the lateral diameter approached the main bore diameter as closely as possible. Another issue that came up with layouts like Jurgens was early exit of the window mill from the whipstock which resulted in the window being shorter than the length intended. It was learned that a way to avoid early exits into the casing and formation was to configure the system to have a restorative force acting on the window mill and acting in a direction normal to the ramp surface. The way this force is generated is a reaction moment from the window mill being deflected by the whipstock ramp and the mill(s) above it being constrained within the tubular. Thus to increase the restraining force on the window mill, it was desirable to have a fairly long watermelon mill. A longer watermelon mill could induce and withstand more bending force and in turn create a greater reaction moment at the window mill that was desired to help the window mill track on the whipstock face.
The problem with a longer string mill was that as it reached the beginning of the whipstock ramp it would get in a bind at the top of the window because of its length and large diameter. While being longer promoted a greater reactive force helping the window mill track on the whipstock ramp to the desired point it made it difficult if not impossible to clear the watermelon mill onto the ramp and out the already made window made by the relatively short window mill.
When the Jurgens system was deployed with mills approximating the drift or clearance diameter of the tubulars, the close coupling of the window and watermelon mill became severely stressed and the connection between those mills experienced failures. The other problem with Jurgens was early exits of the window mill 6 into the formation. This happened because the watermelon mill 14 prevented the restraining force from acting on the window mill 6. As long as watermelon mill 16 was still in the surrounding tubular there was still some restraining force on the mills 6 and 14 to make them track along the whipstock face. However the presence of watermelon 14 substantially diminished the effectiveness of this force allowing the window mill to exit early particularly in soft formation. When watermelon mill 16 got on top of the whipstock ramp the value of that normal restraining force went to zero.
One solution attempted before was to insert a long, smaller diameter tubular between the window mill and a single watermelon mill. While this solved the load transfer issue of having the watermelon mill adjacent the window mill it also provided for an insufficient restorative force on the window mill on relatively long systems which led to short windows being milled as the window mill made an early exit.
The present invention resolves the issue of window length by providing a sufficient restraining force on the window mill through the use of a string mill assembly of sufficient length and diameter to create such a force. At the same time the invention resolves the binding problem as the string mill tilts up on top of the whipstock by providing spaced bearing or cutting structures and a more rounded profile on the lead watermelon mill. As a result at the time the window mill reaches the location where it needs to go out into the formation the reduced diameter shaft between the cutting structures on the bottom hole assembly presents itself at the top of the window to allow the assembly to bend and tilt without getting bound up as the window mill leaves the whipstock. These and other features of the present invention will become more apparent to those skilled in the art from a review of the detailed description of the preferred embodiment, the drawings and the claims which define the full scope of the invention that all appear below.